Insights into deadly coral bleaching could help preserve reefs

April 23, 2013

Coral reefs are stressed the world over and could be in mortal danger because of climate change. But why do some corals die and others not, even when exposed to the same environmental conditions? An interdisciplinary research team from Northwestern University and The Field Museum of Natural History has a surprising answer: The corals themselves play a role in their susceptibility to deadly coral bleaching due to the light-scattering properties of their skeletons. No one else has shown this before.

Using optical technology designed for early cancer detection, the researchers discovered that reef-building corals scatter light in different ways to the symbiotic algae that feed the corals. Corals that are less efficient at light scattering retain algae better under stressful conditions and are more likely to survive. Corals whose skeletons scatter light most efficiently have an advantage under normal conditions, but they suffer the most damage when stressed.

The findings could help predict the response of coral reefs to the stress of increasing seawater temperatures and acidity, helping conservation scientists preserve coral reef health and high biodiversity.

The study of nearly a hundred different species of reef-building corals, including many from the 1893 World's Fair in Chicago, was published this week in PLOS ONE.

"We have solved a little piece of the puzzle of why coral reefs are bleaching and dying," said Luisa A. Marcelino, who led the study. "Our research is the first to show light-scattering properties of the corals are a risk factor."

Marcelino is a molecular biologist and research assistant professor of civil and environmental engineering at Northwestern.

The unusual research involved marine biology, the physics of light transport, the biophysics of how corals handle light and unique technology originally developed for medical applications. The team included Vadim Backman, a physicist and professor of biomedical engineering at Northwestern, and Mark W. Westneat, a coral reef fish biologist and curator of zoology at the Field Museum.

"Coral reefs are like the rain forests of the oceans—the consequences will be catastrophic if coral reefs are lost in great numbers," said Backman, who invented the optical technique used by the team. "Corals are also optical machines. By identifying how much light the skeletons of individual coral species reflect, we have learned which species are more resilient under stress."

Algae provide nutrients to the corals and receive shelter and light for photosynthesis in return. When stressed, the corals can lose their algae. The corals often die of starvation shortly afterward, exposing their white skeletons.

The team used LEBS to measure light transport and light amplification inside the skeletons of 96 different coral species. How fast the light amplification increases with the loss of algae depends on the light transport at the microscale. This was impossible to measure until Backman's low-coherence enhanced backscattering (LEBS) technique became available, which is one of the reasons why this phenomenon has never been studied before.

The specimens were from long-held collections of corals from the Field Museum, including dozens retained from the original Chicago Columbian Exposition and World's Fair of 1893, and the Smithsonian Institution.

The researchers created a family tree of corals that showed bleaching is associated with the physics of light scattering across the entire evolutionary history of corals. Living reef corals are thought to have originated about 220 million years ago, and corals living today are descendants of various branches of these older lineages.

"We found that bleaching and light scattering are associated across the history of reef corals," Westneat said. "This important mechanism occurs repeatedly in all major coral groups, regardless of relationship or evolutionary age."

Corals have evolved to scatter light efficiently. Corals whose skeletons scatter light the most efficiently have an advantage under normal conditions. They also tend to grow faster as this leads to a skeletal structure that is more conducive to scattering.

However, when some of the algae are lost due to stress, the limestone skeletons amplify the light so much that remaining algae have to deal with even more light, thus being at an even greater risk of damage. This creates a vicious cycle forcing more and more algae to leave the coral. Less scattering-efficient corals, on the other hand, do not create the vicious cycle.

More information:
The paper is titled "Modulation of Light-Enhancement to Symbiotic Algae by Light-Scattering in Corals and Evolutionary Trends in Bleaching." This PLOS ONE paper is available at dx.plos.org/10.1371/journal.pone.0061492

Related Stories

Coral specialist Dr. Bert W. Hoeksema of Naturalis Biodiversity Center in Leiden, The Netherlands, recently published the description of a new coral species that lives on the ceilings of caves in Indo-Pacific coral reefs. ...

Scientists have discovered two viruses that appear to infect the single-celled microalgae that reside in corals and are important for coral growth and health, and they say the viruses could play a role in the serious decline ...

A new study by scientists at the University of Miami (UM) Rosenstiel School of Marine & Atmospheric Science shows that corals may be more severely impacted by climate warming when they contain too many symbiotic algae. The ...

Coral reefs are among the most diverse ecosystems on the planet, second only to tropical rain forests. Bird's nest coral (Seriatopora hystrix) is common throughout the Indo-Pacific and is able to live across a range of depths. ...

Coral reefs are predicted to decline under the pressure of global warming. However, a number of coral species can survive at seawater temperatures even higher than predicted for the tropics during the next century. How they ...

Recommended for you

Five million years ago, the Colorado River met the Gulf of California near the present-day desert town of Blythe, California. The evidence, say University of Oregon geologists, is in the sedimentary rocks exposed at the edges ...

Pressure, temperature and fluid composition play an important role in the amount of metals and other chemicals found in wastewaters from hydraulically fractured gas reservoirs, according to Penn State researchers.

Pioneering work being carried out in a cave in New Mexico by researchers at McMaster University and The University of Akron, Ohio, is changing the understanding of how antibiotic resistance may have emerged and how doctors ...

(Phys.org)—A team of researchers with the European Commission's Joint Research Centre and Google Switzerland has combined historical data with modern mapping engines to produce high-resolution maps of the world's surface ...

The ice sheet covering Greenland is four times bigger than California—and holds enough water to raise global sea-level more than twenty feet if most of it were to melt. Today, sea levels are rising and the melting of Greenland ...

0 comments

Please sign in to add a comment.
Registration is free, and takes less than a minute.
Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.